1. Field of the Invention
The invention relates to the field of satellite communication systems, and, in particular, to systems and methods of controlling the bit error rate of transmissions from a terminal to a satellite in order to compensate for rain fade and the like.
2. Description of Related Art
Typical satellite communications systems used to connect remote terminals to other such terminals such as cellular phone systems, or to connect broadcast control center to individual terminals such as used in TV satellite broadcasting, or to control earth observation satellites, all require means to compensate for radio link rain fade and the like. There have been numerous systems for compensating for changes in transmission conditions between a remote terminal and the satellite. An early example can be found in NASA""s ACTS System. In the ACTS System rain fade is estimated at the ground station by monitoring three separate beacons on board the satellite and forward error correction (FEC) rate is adjusted accordingly. However, such a system is only suitable for experimental satellite communication systems, for it takes up to much of the satellites available assets and is not adapted for use with remote terminals. U.S. Pat. No. 5,511,079 xe2x80x9cApparatus And Method For Controlling Forward Error Correction Encoding In A very Small Aperture Terminalxe2x80x9d by D Dillon a system and process that controls the FEC of a VSAT earth station in order to compensate for signal loss between the VAST and hub station due to rain fade and the like. In detail, the hub earth station is adapted to measure the echo signal produced by the satellite""s transponder in response to a transmission therefrom. The hub station is further capable of generating a signal that is transmitted to the VSAT earth stations that is indicative of the propagation conditions at the hub. The VSAT earth station is capable of receiving this signal and in response thereto determines the proper FEC in accordance with the propagation conditions both locally at the VSAT and remotely at the hub. In other words, the VSAT slows or speeds up the data transmission rate depending upon propagation conditions. However, such a system is designed for use with a VSAT that does not have means to measure the quality of a received or transmitted signal. Furthermore, it is not usable where the satellite incorporates on board processing and the uplink and downlink signals are decoupled.
U.S. Pat. No. 5,721,742 xe2x80x9cTerminal, Network, And Communication Systemxe2x80x9d by I, Okajima, et al. discloses a communication system where the data for transmission is FEC encoded based on the type of message and not on the propagation conditions at any point along the link. The decision to apply an FEC is made solely in the VSAT based on a predetermined message type (length) criteria. Information is included in the sent message to allow the receiving terminal to correctly decode the message. Here no attempt is made to compensate for atmospheric conditions, such as rain fade.
U.S. Pat. No. 4,047,151 xe2x80x9cSatellite Communications System And Apparatusxe2x80x9d by P. Baran relates to communication systems where the data for transmission is encoded in a spread spectrum scheme that allows gradual consumption of resources in response to propagation conditions. However, no mechanism for obtaining propagation conditions is provided.
U.S. Pat. No. 4,261,054 xe2x80x9cReal-time Adaptive Power Control In Satellite Communications Systemsxe2x80x9d by H. Scharia-Nielsen discloses a transponded communication systems wherein means are provided to adjust the sending VSATs uplink power based on another VSAT""s reported received detected decode errors. This adaptive power control (not adaptive FEC) is based on the summation of the propagation conditions between the sender and the monitoring VSAT (one or both of which may be in rain) and may not be representative of the sender-to-receiver VSAT path. It does not allow the VSAT to be its own link quality monitor and allowing power and/or FEC adaptation based solely on its VSAT-to-satellite propagation conditions.
U.S. Pat. No. 5,335,249 xe2x80x9cMethod And Apparatus For Spread Spectrum Communicationsxe2x80x9d by T. E. Krueger, et al. discloses a communications system that includes means to select between using narrow band or spread spectrum for a link between a sender and receiver, and to optimize the spread spectrum parameters to meet the link reliability required. The method establishes a link and selects the xe2x80x9cbestxe2x80x9d parameters. When the link degrades below the reliability threshold the process is reset to begin again. However, this invention does not use a dynamic adaptive coding/power process in response to changing link propagation conditions.
U.S. Pat. No. 5,603,096 xe2x80x9cReverse Link, Closed Loop Power Control In A code Division Multiple Access Systemxe2x80x9d by K. S. Gilhousen, et al. discloses a communications systems where the receiver measures the sender""s power and compares it to a stored expected value. The receiver then instructs the sender to increase/decrease its transmitter level to stay within a given received power level window. This invention does use received power measurements but does not provide for the sender to determine its own power needed in response to local link propagation conditions.
U.S. Pat. No. 4,309,764 xe2x80x9cTechnique For Increasing the Rain Margin Of A Satellite Communication Systemxe2x80x9d by A. Acampora applies to transponded communications systems where a VSAT uses an unspecified mechanism to adjust its uplink power to overcome rain fades and notifies the Hub to notify the sender to use additional return path resources to overcome a downlink fade. The Hub controls the assignment of resources. In this system use of additional assets to counter rain fades is applied to both VSAT-to-satellite links, even if only one link is experiencing a rain fade.
U.S. Pat. No. 5,768,684 xe2x80x9cMethod And Apparatus For Bi-Directional Power Control in A Digital Communication Systemxe2x80x9d by J. E. Grubb, et al. discloses a processing satellite system where the receiving nodes (including the satellite) measure the received power level from a sender using an estimated bit error rate. This received power level is then sent back to the sender such that the sender can adjust its transmitting power level to match the link propagation conditions. The sender does not perform its own uplink and downlink power level measurements and does not include adaptive coding to increase the overall link margin dynamic range.
U.S. Pat. No. 5,699,365 xe2x80x9cApparatus And Method For Adaptive Forward Error Correction In Data Communicationsxe2x80x9d by J. T. Klayman, et al. discloses a communications system where the receiver monitors the FEC error results from messages sent to it. When the errors reach a threshold, the receiver informs the sender to use a more/less robust FEC. However, the same FEC is employed from the sender to the receiver, and this wastes band width on portions of the link not needing a robust FEC.
Therefore, what is needed is an active closed loop adaptive coding system between the satellite and the VSAT without using any Hub or network control center (NCC) measurement data in the process. Furthermore, it is also important to permit the adjusting of both the VSAT""s uplink and downlink FEC rates and uplink power to meet propagation conditions local to the VSAT-to-satellite link only, thus allowing higher throughput in both directions without using increased satellite resources. The NCC""s local propagation conditions should remain immaterial and only independently operated on its own Hub-to-satellite link.
Thus, it is a primary object of the invention and method of providing a terminal to satellite communication system that can compensate for changes in propagation conditions therebetween.
It is another primary object of the invention to provide a system and method of providing an adaptive coding system for insuring error free communications between a satellite and remote terminal when propagation conditions therebetween deteriorate.
It is a further object of the invention to provide and method of providing an adaptive coding system for insuring error free communications when between a satellite and remote terminal that only requires the determination of local propagation conditions.
Finally it is an objective of the invention to not increase satellite complexity by requiring satellite measurements of the VSAT-to-satellite link parameters.
The invention is a method and system for determining the uplink transmission quality of a signal sent from a terminal to a satellite, which may vary due to changes in atmospheric conditions. In detail, the method includes the steps of periodically preparing a self-addressed data packet for sending to the satellite for transmission to the satellite. These self-addressed data packets are sent to the satellite dispersed within the normal data packets. However, this particular data packet is transmitted to the satellite at an incrementally reduced power setting. The satellite thereafter retransmits the data packet back to the terminal. The transmission quality of the retransmitted self-addressed data packet is measured and compared to a reference transmission quality level. The difference, if any, is used to generate an error signal indicative of the required change in transmission quality level of the terminal uplink signal required to maintain the uplink signal at a specific level.
If the transmission quality level is unacceptable, a determination is made as to whether the terminal can increase the power of its transmissions. If it can, the power level of the terminal is incrementally increased. This procedure is repeated until the transmission power level of the self-addressed data packets is within acceptable level power. Thereafter, the transmission power level of all data packet transmissions is raised to this level. However, if the transmission quality level of these self-addressed data packets is above an acceptable, then the power level is reduced in incremental steps, until the quality level is at a just acceptable level. Thereafter, the transmission level of the all the data packets is reduced to this level. It should be noted that such power levels occur over a relatively long time as defined by local rain fade rate. The frequency of power adjustments is based on the rate of change in propagation conditions.
However, there is a point where the transmissions between the terminal and satellite have been so degraded that the power level of the terminal can no longer be increased. At this point in time, the terminal automatically requests the network control center to switch the satellite to receive uplink signals transmitted by the terminal from light encoding rates to heavy encoding rates. The satellite then switches so that it can receive and process heavy encoding rates rather than light encoding rates. Thereafter, the terminal transmits data packets at these heavy encoding rates. Of course, the power transmission adjustment procedure continues. If, on the other hand, the terminal has been transmitting at heavy encoding rates, and transmissions conditions improve to a point that the power level can not be further reduced, the opposite occurs. The terminal requests the NCC to change from heavy encoding rates to light encoding rates.
The above method requires that the terminal be transmitting in order to make a determination of the required terminal transmission power level and/or encoding rate. However, it is possible to determine the required power level and encoding rate, by use of terminal uplink time synchronization probes. A part of each data channel contains a time probe signal slot that is used to insure that the terminal is in synchronization with the satellite in a system using Time Domain Multiple Access (TDMA) Mode. Here specific timing probe signal are periodically prepared and transmitted to the satellite at a specific power level. The satellite uses these signals to determine if the terminal is time synchronized with the satellite and will inform the terminal if the signal is early or late. It will also inform the terminal whether is has received the timing probe. If the terminal is informed that it has not received the signal, it incrementally increases the power output of the terminal when transmitting the specific timing probe signal if the previously sent specific timing probe was not received. This procedure is repeated until the satellite detects the probe and reports that the timing probe signal has been received. At this point in time, the power level all the data packets is raised to this level. The data packet uplink power setting is coorellatable to the timing probe uplink power. Then the power level of the terminal used to transmit these specific timing probes is decreased until the satellite reports that it no longer is receiving these specific timing probes. Thereafter, the power level is raised back up towards the previous level where the satellite had reported receipt of the specific timing probe signal and all data transmissions from the terminal are made at this power level. This process is repeated with hysteresis allowing the terminal to track a varying satellite receive threshold.
However, as previously mentioned, there is a point where the transmissions between the terminal and satellite have been so degraded that the power level of the terminal can no longer be increased. At this point in time, the terminal automatically requests the network control center to switch the satellite to receive uplink signals transmitted by the terminal from light encoding rates to heavy encoding rates. The satellite then switches so that it can receive and process heaving encoding rates rather than light encoding rates. Thereafter, the terminal transmits data packets at these heavy encoding rates and timing probes using a correspondingly longer code. Of course, the power transmission adjustment procedure continues. If, on the other hand, the terminal has been transmitting at heavy encoding rates, and transmissions conditions improve to a point that the power level can not be further reduced, the opposite occurs. The terminal requests the NCC to change from heavy encoding rates to light encoding rates.
The use of timing probes has the advantage of being able to determine the proper power level even when data is not being transmitted by the terminal. As long as the terminal is on, timing probe signals are generated in order to maintain the terminal properly synchronized with the satellite the terminal then knows the correct data packet uplink power. The method can be used as a separate method of insuring minimum use of terminal power, or in conjunction with the use of self-addressed data packets.
The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which the presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention.